The present invention relates to a fuel control apparatus of an engine, and particularly to an apparatus for controlling an amount of fuel supplied to the engine correspondingly to an intake pressure.
Japanese Unexamined Patent Publication No. 59-15656 has disclosed a fuel control apparatus of an engine wherein a fuel injector is provided in an intake passageway, an intake pressure sensor for detecting the intake pressure is provided in the intake passageway on the downstream side of a throttle valve and an amount of the fuel supplied from the fuel injector is controlled on the basis of the intake pressure detected by the intake pressure sensor and the number of rotations of the engine.
In general, there is known a technique that the intake passageway of the engine is provided with means for changing a resistance of intake air flowing therethrough so that output characteristics of the engine can be improved. By way of example, there are known the following two intake resistance changing means.
According to the first intake resistance changing means, the engine has first and second branch passageways provided in parallel with each other in the intake passageway thereof. The second branch passageway has a control valve so as to change the area of the intake passageway depending on opening or closing thereof. When an engine speed is decreased, the control valve is closed so that the intake air is supplied through only the first branch passageway. Consequently, the intake air is easily flowed to promote combustion. On the other hand, when the engine speed is increased, the control valve is opened so that the intake air is supplied through both the first and second branch passageways. Consequently, the resistance of the intake air is reduced, so that the amount of the intake flow is increased. Thus, an output of the engine is increased.
According to the second intake resistance changing means, the engine has high and low speed intake passageways provided in parallel with each other in the intake passageway thereof. The low speed intake passageway has a greater equivalent tube length. The high speed intake passageway has a smaller equivalent tube length and provided with a control valve. The length of the intake passageway is changed depending on the opening or closing of the control valve. When the engine speed is decreased, the control valve is closed so that the intake air is supplied through only the low speed intake passageway. Consequently, there can be obtained an intake inertia effect corresponding to the length of the low speed intake passageway. On the other hand, when the engine speed is increased, the control valve is opened so that the intake air is supplied mainly through the high speed intake passageway. Consequently, there can be obtained the intake inertia effect corresponding to the length of the high speed intake passageway. Thus, the output is increased the whole rotating area of the engine.
However, in the cases where the above-mentioned intake resistance changing means are provided on the above mentioned apparatus for controlling an amount of fuel supplied to an engine correspondingly to the intake pressure, an air fuel ratio is changed when the intake resistance is changed along with the change in area or length of the intake passageway. For example, in the case where the engine speed is increased with acceleration, the resistance of the intake air flowing through the intake passageway is reduced if the area of the intake passageway is increased or the length of the intake passageway is reduced. Consequently, when the opening of the throttle valve is not changed, the intake flow is excessively increased. When the opening of the throttle valve is slightly reduced in order to make the intake flow proper, the intake pressure is reduced. As a result, the intake flow does not correspond to the intake pressure. Therefore, when a supply amount of the fuel is conventionally and uniformly controlled correspondingly to the intake pressure, the air fuel ratio is deviated from a desired value. Also in the case where the engine speed is decreased with deceleration, the air fuel ratio is deviated from the desired value.